Global warming is caused by greenhouse gases in the lower atmosphere. The Earth is warmed by absorbing sunlight and cooled by emitting blackbody radiation in the far infrared (IR) out into space. The greenhouse gases reduce the amount of IR radiation leaving the Earth. Hence, the Earth warms more in the presence of greenhouse gases, emitting more IR radiation, until the amount of IR radiation leaving the Earth is again in balance with the amount of sunlight energy arriving.
Most attempts to reduce global warming focus on reducing the amounts of the greenhouse gases (CO2, Methane, etc) in the lower atmosphere back to levels seen before 2005 or even back to levels seen in 1950. Unfortunately, even if we were to stop burning fossil fuels now and greatly reduce the amount of CO2 entering the atmosphere, the high level that is already there would last for something like 1000 years.
Therefore, alternate approaches, collectively called geo-engineering, have been proposed, in which something is introduced into the atmosphere to counteract the effects of the greenhouse gases. In one of these, a sulfate-based aerosol, made up of water droplets containing sulfuric acid, is introduced into the upper atmosphere, where the droplets will scatter sunlight and reflect some of the sunlight back into space. Since the total amount of sunlight that would reach the Earth is diminished, the Earth will cool, even through the CO2 is still trapping some of the IR radiation produced at the Earth's surface from leaving the Earth.
Unfortunately, placing huge quantities of sulfate-based aerosols into the upper atmosphere is highly controversial for several reasons. First, the sulfates will come down from the upper atmosphere in the form of acid rain. Acid rain is well known to have detrimental effects on trees, food crops and other plant life. Second, the droplets formed by adding sulfates to water are very small, in the range of 0.01-0.25 μm, with most around 0.05 μm. Particles of this size scatter short wavelength visible light more effectively than other parts of the sunlight spectrum. Both red and blue wavelengths of light are required by plants for photosynthesis. Scattering more of either red or blue could slow the growth of plants. It could adversely affect food production and slow the rate at which trees and other plants consume CO2. Finally, the time for which the SO2 water droplets will remain in the upper atmosphere is not known. There is a tendency for them to bump into each other and agglomerate. When big enough, they will fall more quickly down to Earth. If their lifetime in the upper atmosphere is too short, then more SO2 will be required to replace them and more acid rain will be created.
In another geo-engineering approach, it has been suggested that particles composed of metal oxides, like Al2O3, be introduced into the top of the lower atmosphere, from 7-13 km above the Earth, where the CO2 in the atmosphere tends to accumulate. These particles have much larger diameters ranging from 5 μm-10 μm.
A part of this approach is for these metal oxide particles to absorb the near IR (0.9 μm-2 μm) and re-radiate it back out into space. By doing so, about 30% of the energy in sunlight would not reach the surface of the Earth, causing the Earth to cool. Unfortunately, metal oxide particles, such as Al2O3 or Thorium Oxide particles that have been suggested for this approach, are quite transparent in the near IR and would not absorb much sunlight in this range of wavelengths. Since they absorb 10 μm wavelengths very well, they act like CO2, Methane and other greenhouse gases. They will reflect the black body radiation near 10 μm back to Earth and result in warming the Earth.
Another part of this approach is to use the Weisbach effect, where a material like Al2O3 absorbs energy at a wavelength of 10 μm and re-radiates it out as visible light. Gas lantern mantles use this effect to absorb the heat from burning gas and radiate a bright, white light. As Welsbach materials, Al2O3 or Thorium Oxide could absorb the 10 μm, black body radiation and re-radiate it as visible or ultra-violet light. As visible light, this energy would not be stopped by the CO2 and other greenhouse gases.
However, for this approach to be effective, these particles would have to be positioned within or below the CO2 layer. The black body energy from the Earth would have to hit these particles before it hit the CO2 layer. This would require that the particles are dispersed in the lower atmosphere at about 7 km, and certainly below 10 km. Unfortunately, in the lower atmosphere, particles in the air are quickly washed out of the air by rain. Particles with diameters in the range of 5 μm-10 μm fall out very quickly. Moreover, the cost of this approach would be high. One would have to mine relative pure Al2O3 or other metal oxides, and grind them into particles of the desired size.
Finally, people have also proposed introducing more water vapor into the upper atmosphere. Unfortunately, the water vapor tends to agglomerate readily to form large particles (ice crystals) of water. These ice crystals are commonly seen in the high Cirrus clouds that form in the upper part of the lower atmosphere. Water vapor and ice crystals are among the strongest greenhouse gases. While they might be effective in reflecting some sunlight back into space, their net effect is to reflect more heat back to the Earth.
There is, therefore, a need to identify and provide a different material from those previously considered, that could be positioned in the upper atmosphere to more efficiently provide the increased albedo effect desirous for reducing global warming, without incurring the risk of acid rain, negatively impacting photosynthesis, or increasing the reflection of IR radiation back to the Earth's surface.